Frothing a liquid involves supplying a gas like air to the liquid and mixing the liquid with the gas, for the purpose of obtaining a mass of bubbles in the liquid, which is referred to as foam. In many cases, the liquid to be frothed is milk. Especially in the field of making coffee and coffee specialties, there is a need for a device which is capable of frothing milk in a user-friendly manner. It is a well-known option to combine a frothing process of milk with a heating process of milk, wherein air is supplied to the milk for obtaining foam, and wherein steam is supplied to the milk for increasing the temperature of the milk.
In general, it is known to supply air to be used in a frothing process of a liquid through a hole or a channel to a space where the frothing process is intended to take place. It appears in practice that such a hole or channel always gets contaminated as time passes, assuming frequent use of the device in which the frothing process takes place. In many cases, the hole or channel is relatively narrow, which renders it sensitive to contamination, wherein it is especially vulnerable to clogging with milk fats, milk stone and bacterial growth. Moreover, the hole or channel is difficult to clean as a result of a small cross-sectional area. For sake of completeness, it is noted that the inside of the hole or channel is reached by the milk under the influence of inevitable pressure fluctuations occurring during practical use.
Applicant proposes a way of solving the problems as mentioned in the foregoing, which is the subject of European patent application 10166464.7. In particular, Applicant proposes the use of a device in which the supply of gas for the purpose of frothing is not realized by using a hole or a channel providing access to a space where the frothing is supposed to take place. Instead, in the device proposed by Applicant, there are two separate spaces, wherein one of the spaces is arranged such as to receive content from another of the spaces, and wherein a free space is present between the inlet of the one space and the outlet of the other space. As a result, gas that is present in the free space as mentioned is free to enter the spaces through their openings. A flow of gas can be obtained under the influence of a flow of liquid from one space to another, wherein the gas is taken along by the liquid as a secondary flow. In any case, the openings of the spaces and the free space between the inlet of the one space and the outlet of the other space are considerably larger than conventional holes or channels for supplying air, so that clogging is virtually impossible.
In the device proposed by Applicant, the intake of frothing gas is not precisely controlled, and because of the fact that the intake is larger than in conventional situations, the amount of frothing gas is too much for the amount of liquid to be frothed. This results in an excess of air that is present in the frothing chamber. This excess of air has to be separated from the main flow of frothed milk and removed from the chamber through the outlet tube. The excess of air easily leads to negative effects such as too coarse foam on unstable and unpredictable running behavior of the milk froth.